Many products are made from or contain thermoplastic elastomeric polymers in various elastic components that provide desirable elastic properties, often at a relatively low cost. One particularly useful class of thermoplastic elastomeric polymers are A-B-A' elastomeric block copolymers where A and A' are each a thermoplastic polymer endblock and where B is an elastomeric polymer mid-block. Typically, the elastomeric polymer mid-block is a conjugated diene monomer unit.
However, polymers having a polydiene mid-block tend to be poorly suited for the demanding conditions encountered in certain modern extrusion processes. These polymers may degrade oxidatively if held for a long period of time at high temperatures in the presence of oxygen. Also, products made from these polymers may be placed in environments that can cause the polymers to chemically break down. In order to increase their thermal stability and resistance to oxidation, these elastomeric block copolymers may be hydrogenated to produce copolymers having a low degree of residual ethylenic unsaturation in the polydiene mid-block. Such polymers have essentially saturated olefin midblocks which tend to be thermally stable and resistant to oxidative degradation. Exemplary elastomeric block copolymers having saturated olefin midblocks are the Kraton.RTM. G series rubbers available from the Shell Chemical Company.
Those elastomeric block copolymers can be formulated into extrudable compositions, often with added stabilizers and antioxidants, that are especially well suited for the severe conditions of extrusion processes such as, for example, meltblowing processes. Large quantities of these extrudable compositions are processed into elastic nonwoven webs that are incorporated in a variety of disposable products, including disposable personal care products such as, for example, disposable diapers, disposable work wear and other disposable garments.
Unfortunately, the polymer modifications that provide thermal stability and resistance to oxidation also hinder chemical breakdown of the polymer after the products have been used and disposed of in a composting system. Thus, while some extrudable elastomeric materials will withstand severe processing conditions and, once incorporated into a product, will survive environments that could cause the polymer to chemically break down, there is still a need for an extrudable elastomeric material which will meet those needs and will still break down under typical compost conditions.
When present in certain polymers in sufficient amounts, some transition metals are known under the appropriate conditions to initiate chemical reactions that degrade the physical properties of those polymers. For example, U.S. Pat. No. 3,332,926 issued Jul. 25, 1967 to Baron et al., discloses a process for making a low-molecular weight polyolefin by mixing a high molecular weight polyolefin with a metal salt of a carboxylic acid and then applying heat. According to the patent, from about 0.075 percent to 10 percent, by weight, of a metal salt of a carboxylic acid may be used. It is disclosed that the resulting mixture is heated in an atmosphere that is substantially free of oxygen to a temperature from about 275.degree. to about 450.degree. C. According to the patent, useful metal salts of carboxylic acids include sodium stearate, calcium stearate, magnesium stearate, zinc stearate, sodium oxalate, sodium citrate, sodium acetate, sodium tartrate, and potassium acid phthalate.
U.S. Pat. No. 3,941,759 issued Mar. 2, 1976 to Taylor, et al., discloses a degradable plastic that contains an organic photosensitizer and at least one organic derivative of a transition metal. According to the patent, controlled degradation of the plastic is initiated by photo-oxidative reaction of the organic photosensitizer. This photo-oxidative reaction is sustained by the organic derivatives of the transition metals. It is disclosed that the compositions described in that patent will gradually oxidize in the dark after an initial short-term exposure to ultraviolet light. Suitable organic photosythesizers are disclosed as including ketones, azo compounds such as azobenzene, organic dyes, and other compounds such as benzophenone. The organic derivatives of transition metals are disclosed as including naphthanates, oleates, acetates, stearates, benzoates, citrates, and similar compounds derived from cobalt, chromium, iron, nickel, manganese, and other transition metals.
U.S. Pat. No. 3,994,855 issued Nov. 30, 1976 to Boberg, discloses a polymer composition which is degradable under the action of ultraviolet light and/or sunlight, and which also may be subject to thermal degradation. According to the patent, thermoplastic polymers or copolymers of .alpha.-olefins may be made photosensitive and degradable by employing one or more transition metal compounds.
U.S. Pat. No. 4,931,488 discloses thermoplastic polymer compositions, especially .alpha.-olefin polymers that include a biodegradable substance such as, for example, starch, a transition metal compound, and a fatty acid or fatty acid ester. These polymer compositions degrade under the action of heat and/or ultraviolet light. According to the patent, one or more other transition metal compounds may be added to catalyze the degradation.
U.S. Pat. No. 4,360,606 issued Nov. 23, 1982 to Tobias, et al., discloses photodegradable polymer compositions. According to the patent, a degradable polymer may be made which contains at least one photosensitive aliphatic or aromatic ketone and at least one readily auto-oxidizable olefinic compound having an amine with at least one olefinic group.
U.S. Pat. No. 4,461,853 issued Jul. 24, 1984 to Gilead, et al., discloses a controllably degradable vinyl polymer composition which contain a complex of two different metals. According to the patent, a combination of iron and nickel compounds provides an unexpected enhancement in the photo-degradation of vinyl films.
PCT International Publication No. WO 88/09354 discloses a degradable polymer composition which includes a blend of normally stable chemically saturated polymer, a less stable chemically unsaturated polymer or copolymer, an anti-oxidant active over a limited period of time and a latent pro-oxidant, such as an organic salt of a transition metal compound. According to the patent, the normally stable chemically saturated polymer may be a polyolefin such as polyethylene. The less stable chemically unsaturated polymer is disclosed as being a styrene/butadiene block copolymer or natural rubber that provides adequate levels of autoxidizable unsaturation. The transition metal salts are disclosed as being organic acid salts of metals such as cobalt, manganese, or copper. The organic acids are described as having sufficiently high molecular weights to yield metal salts that are soluble in the polymer blends.
While these references describe various degradable polymers, they do not address problems related to providing extrudable elastomeric materials that will withstand severe processing conditions and, once incorporated into a product, will survive environments that could cause the polymer to chemically break down, and yet will still break down under typical compost conditions.